The present invention relates to induced polarization mineral prospecting and more particularly to improved apparatus and methods for removing noise components from the signals detected in induced polarization prospecting.
The basic methods and apparatus used for induced polarization mineral prospecting are well known and have been in use for many years. For example, U.S. Pat. No. 3,967,190 issued to Zonge, provides a general description of the older techniques, together with more detailed descriptions of more modern signal processing methods. Most modern improvements to the process have involved efforts to discriminate between various types of minerals and to increase the depth of exploration. The Zonge patent deals primarily with signal processing techniques which allow particular plots of induced polarization data to be generated for comparison to models to aid in discrimination of various minerals.
As with any other type of signal processing system, accuracy depends upon the signal-to-noise ratio of the recorded information. For shallow prospecting, the signal-to-noise ratio is usually quite high. However, as the exploration depth is increased, the signal levels are reduced rapidly. The signal level can, of course, be improved by increasing the level of current input, but this approach has definite limits. At the greater search depths, electromagnetic coupling or dispersion problems require use of lower frequency input signals. The telluric noise level increases at these lower frequencies further reducing the signal-to-noise ratio. Thus, as search depth is increased, the cancellation of telluric noise becomes more important.
The above referenced Zonge patent teaches the use of a variable frequency transmitter to provide induced polarization data over a range of frequencies. This spectral data is intended to provide information identifying the type of material detected, e.g. sulphides or clay. All frequencies used in this process must of course be below the frequency at which dispersion degrades the signals. Thus, if a deep looking spectral survey is run, very low frequencies must be employed and the telluric noise becomes an even more serious problem.
It is well known that naturally occurring earth currents, known as telluric currents, are responsible for a major portion of the noise normally encountered in any type of induced polarization prospecting method. Various efforts have been made in the past to provide additional electrodes in an induced polarization electrode spread for providing an indication of the telluric noise level so that a correction factor may be generated for improving the signal-to-noise ratio of the recorded induced polarization signals. A typical approach has been to lay out a linear spread of electrodes along a survey path with a portion of the electrodes being used as current inputs and sensing electrodes during each step of the induced polarization prospecting process. Electrodes on opposite ends of the spread are often employed together with one additional lateral or offset electrode for recording telluric noise indications. In such an arrangement, the telluric noise record generally includes a fairly large component of the induced polarization input signal which must usually be removed before the noise record is used to generate a correction factor for the desired signal records. If such corrections are not made, the use of the recorded noise to correct the desired signal records may cause distortion of the signals. It is for this reason that fairly complicated processing programs have been developed for correcting the noise record by processing both in-line and normal telluric noise records. Such processing obviously increases the complexity of the prospecting systems, may delay the application of the correction factor, and may degrade the overall accuracy of the prospecting system.
Thus, it is seen that it is desirable to provide records of telluric noise occurring along an induced polarization prospecting path with such records being substantially free of induced polarization signal components. In addition, it is desirable that such signals be acquired in a simple manner and be in such form that they may be applied to correction of desired induced polarization signals without further processing.